Comey told a Senate committee Thursday that he believes Trump fired him over the Russia probe, and he accused the White House of lying about the details of the dismissal. He also admitted that he had leaked to the press memos describing his talks with Trump, saying he hoped the stories would spur the appointment of a special counsel to take over the investigation of the Trump campaign's contacts with Russia.

The former FBI director also appeared to confirm Trump's statements that, on three occasions, Comey told the president he was not personally under investigation with regard to Russia.

While Trump's attorney issued a statement defending Trump and attacking Comey for the leak, and Trump's son Don Jr. tweeted throughout the hearing, the president himself remained silent on the subject and did not tweet all day.

That changed early Friday.

One source of the Trump-Comey dispute: Comey's memos.

The former FBI director said he he kept notes on his meetings with the president because he was concerned Trump might lie about the nature of their conversations.

After his dismissal, Comey testified to the Senate Intelligence Committee, he told a friend about his memos and asked him to leak the information about them to reporters.

Trump and aides pounced on that revelation, accusing Comey of improperly leaking privileged conversations. They also disputed Comey's assertion that Trump asked the FBI director for a pledge of personal loyalty to the president.

The president and his aides backed other parts of Comey's testimony, however, including portions where the then-FBI director told the president he was not personally under investigation over Russia.

The FBI is investigating links between associates of Trump during last year's campaign and Russians who sought to influence the election by hacking Democrats. Comey said that Trump asked him specifically whether he could drop the investigation with respect to former National Security Adviser Michael Flynn, who Trump fired for withholding information about his contacts with foreign governments.

The president's critics said Comey's claims that Trump asked him about dropping the Russia investigation could amount to obstruction of justice.

"What struck me is that it is completely unacceptable for the president to ask the FBI director to be personally loyal to him and to ask the FBI director to drop it with regards to an ongoing investigation into his former national security adviser,” said Sen. Chris Coons, D-Del.

Democratic Sen. Brian Schatz of Hawaii fired back at Trump on Twitter Friday morning, saying the president appeared to be accusing Comey of a serious crime: lying to Congress.